Emacs 24

As a binary package

MacPorts

to install Emacs 24. The MacPorts installer updates your PATH environment variable, so the MacPorts build of Emacs should be found by your shell before the (outdated) build that comes with Mac OS when you type 'emacs'. If you prefer the Emacs.app build, that's available by executing:

$ sudo port install emacs-app

Homebrew

Emacs 24 is also available via homebrew. Just type the following incantation in your shell and you’re done:

Emacs 23

Aquamacs

A near-OS-X-native port of GNU Emacs, integrating Emacs with all the features of Mac OS X. Available here. As of this writing, the latest version (2.4) is based on Emacs 23.3. Comes bundled with some handy packages like AUCTeX.